In a conventionally known system disclosed, for instance, by Japanese Patent Laid-open No. Hei 4-359142, an oxygen sensor is mounted in an internal combustion engine exhaust path to detect the status of an exhaust gas. The oxygen sensor generates an output in accordance with the oxygen concentration in the exhaust gas after reaching its activity temperature. To this end, the oxygen sensor incorporates a heater, and heated to its activity temperature by the heater while the internal combustion engine is operating.
The exhaust gas contains a large amount of water vapor. Therefore, if the oxygen sensor temperature suddenly drops after an internal combustion engine stop, a large amount of water is adsorbed by a sensor element of the oxygen sensor. This water adsorption may give a thermal shock to the sensor element, thereby damaging the sensor element. Therefore, the above-mentioned conventional system continuously heats the oxygen sensor for a period of approximately 5 seconds after an internal combustion engine stop.
When the oxygen sensor is continuously heated for a period of approximately 5 seconds after an internal combustion engine stop, the sensor temperature does not suddenly drop. As a result, the proportion of water remaining in the exhaust path that is adsorbed by the sensor element significantly decreases. As such being the case, the above-mentioned conventional system can improve the durability of the oxygen sensor by restraining the adsorption of water by the oxygen sensor at the time of an internal combustion engine stop.
Including the above-mentioned document, the applicant is aware of the following documents as a related art of the present invention.
[Patent Document 1]
Japanese Patent Laid-open No. Hei 4-359142
[Patent Document 2]
Japanese Patent Laid-open No. Hei 8-75695
[Patent Document 3]
Japanese Utility model Laid-open No. Hei 6-58359
[Patent Document 4]
Japanese Patent Laid-open No. Hei 1-257739
After internal combustion engine startup, the internal combustion engine exhaust sensor is generally heated to a predetermined activity temperature. In such a heating process, the output from the exhaust sensor temporarily deviates from normal due to the influence of adsorbable species on the sensor element. It is assumed that the adsorbable species becomes chemically adsorbed to the sensor element when the exhaust sensor temperature lowers after an internal combustion engine stop. The deviation of the exhaust sensor output due to the influence of the adsorbable species increases with an increase in the amount of adsorbable species adsorption.
The applicant of the present invention has found that the amount of adsorbable species adsorption to the exhaust sensor greatly depends on the exhaust sensor temperature and ambient temperature prevailing after an internal combustion engine stop. More specifically, it is found that if the exhaust sensor temperature lowers to reach a temperature region in which the adsorbable species may become chemically adsorbed (hereinafter referred to as the “absorption temperature region”) before the exhaust path temperature sufficiently lowers, a large amount of adsorbable species readily becomes adsorbed.
According to the system disclosed by Japanese Patent Laid-open No. Hei 4-359142, the oxygen sensor is continuously heated for a period of approximately 5 seconds after an internal combustion engine stop. That is, the system has a function for delaying the temperature of the exhaust sensor (oxygen sensor) lowering into the adsorption temperature region while the process for lowering the exhaust path temperature is in progress.
However, the above-mentioned conventional system stops to heat the oxygen sensor before the exhaust path temperature sufficiently lowers. More specifically, the exhaust path temperature does not significantly lower during the 5 second in which the above conventional system continuously heats the oxygen sensor. Therefore, the above conventional system cannot decrease the amount of adsorbable species adsorption. Consequently, the above conventional system cannot restrain the exhaust sensor output from deviating from normal under the influence of the adsorbable species.